An engine (stratified scavenging two-cycle engine) is conventionally known in which lead air that has been introduced in advance into a scavenging channel and a subsequent air-fuel mixture flow in a stratified manner from a scavenging port into a cylinder, whereby the non-combusted gas can be prevented from flowing out from an exhaust port (blow-bye can be prevented) during a scavenging stroke of a two-cycle engine.
A variety of systems for introducing the lead air into the scavenging channel are employed in stratified scavenging two-cycle engines. With the most basic configuration, an external air introduction path having a lead valve is connected to the scavenging channel, and the external air (lead air) flows in from the external air introduction path into the scavenging channel due to the pressure reduction in the crank chamber in the compression stroke.
Patent Document 1: Japanese Patent Application Laid-open No. 10-121973.